The battlefield of the future may contain laser weapons used to blind the crew of combat vehicles. These future weapons will probably fire laser pulses a few nanoseconds long and will have very high peak power intensities. When such a laser pulse enters the human eye, it is focussed to a very small spot on the retina and burns the retina. These weapons are expected to be "agile," or able to lase unpredictably at any color of light in the visible spectrum, so that simple colored absorbing glass or reflection filters are not viable for protecting the eye. That is, if we use filters to block red laser light, green lasers could blind us and vise versa. Obviously, protective devices absorbing light throughout the visible spectrum are not a viable option either, since no spectra would then be available for vision. To neutralize future laser threats, a material or device acting independently of wavelength is needed which passes low energy light (such as normal scene illumination) to the eye while blocking high energy light. A device or material operating in this fashion is said to be nonlinear in energy. A vision system using a nonlinear absorbing or scattering material is described in my U.S. Pat. No. 5,345,340 issued Sep. 6, 1994.
One concept for nonlinear laser protection is a suspension cell limiter such as a carbon black suspension cell (CBS). In a CBS, nonlinear blocking is achieved by focussing incident light into a cell containing a suspension of carbon particles. The energy absorbed by any carbon atom is greater than the work function of an electron within the atom so that the electron escapes, thereby causing a free electron and a C+ ion. A collection of these electrons and ions is called a plasma. Plasmas can be very energetic and dense and can be very strong absorbers or reflectors of photons. Once the beginning of an incident laser pulse has initiated a plasma, the plasma absorbs and scatters the rest of the pulse. Additionally, heat from the plasma vaporizes the solvent and the resultant bubbles act to scatter light incident on the CBS.
Another nonlinear laser protection method uses sacrificial mirrors. Sacrificial mirrors are mirrors which damage easily and oblate (blow away) when high energy light strikes them, whereby less of the laser pulse is reflected to the eye. Still another concept involves the use of nonlinear absorbers, which have an electronic structure whose ground state is not highly absorbing to visible wavelengths but whose excited state is very strongly absorbing to visible photons. In nonlinear absorbers, low densities of photons (from background illumination) do not pump enough electrons into highly absorbing exited states to effect appreciable absorption. However, the high photon density of the beginning of a laser pulse drives enough electrons to the exited states to absorb the rest of the pulse.
The phenomena occurring during nonlinear absorption or scattering of laser light are extremely rapid. For example, a laser light pulse is typically 10 nanoseconds long, and it is believed that plasmas are formed within a few nanoseconds of a laser pulse becoming incident on a CBS. In order to study these phenomena, it is very helpful And perhaps necessary to measure various facets of their temporal and spatial dynamics. Such facets include: time needed for plasma initiation, time needed for bubble initiation, energies needed for these initiations, the shape and rate of bubble or plasma growth over time, and length of bubble and plasma life. Pioneering work in measuring spatial and temporal dynamics has been done by Franco Docchio, Pietro Regondi, Malcom R. Capon and John Mellerio and published in their article, "Study of the Temporal and Spatial Dynamics of Plasmas Induced in Liquids by Nanosecond Nd:YAG Laser Pulses. 1: Analysis of Plasma Starting Times," Applied Optics, Vol 27, No. 17, September 1988. Docchio et. al. used a flashlamp and steak camera to obtain a framing rate of 20.times.10.sup.6 frames per second.